The present invention is concerned with controlling the softening point and for providing a preselected softening point of solder glass, and in particular, is directed to providing a preselected softening point of solder glass containing copper oxide.
Solder glass, as the name indicates, is glass used to join together other glasses and/or other materials. Solder glass must have a softening point lower than that of the glasses which it joins together and must have substantially the same thermal coefficient of expansion. Accordingly, one of the most important properties of a solder glass is its softening point. However, great difficulty has been experienced in controlling the softening point of solder glass including those solder glasses which contain copper oxide. Such glasses are of interest because copper additions are known to change the softening point of such glasses without perturbing significantly their coefficients of thermal expansion. Even when different batches of similar compositions are reacted to provide a particular solder glass, varying softening points have been obtained. Furthermore, on occasions the softening point has been so inconsistent that production of the solder glass has had to be shut down. Accompanying such variations in addition, undesirable precipitation of cuprous oxide may occur.
It is known that the softening point of solder glass can be altered by varying the amount of copper oxide employed in the starting composition without affecting the coefficient of thermal expansion of the final glass to any significant degree, e.g., see U.S. Pat. Nos. 3,088,833, 3,088,835, 3,127,278, 3,291,586, and 3,564,587. However, the art has not heretofore recognized any specific relationship between the copper oxide concentration and the softening point of the solder glass. Heretofore it was generally felt that the effect of copper oxide on the properties of a copper-containing glass was related to the total copper concentration rather than any particular form in which the copper existed in the composition; for example, see L. C. Hoffman et al., "A Survey of the Effect of Composition on the Internal Friction of Glass", Glass Ind., 38 (2) 81-85, 104-5 (1957).
In fact, much of the recorded work on copper-containing glasses does not even deal with solder glasses and is primarily concerned with qualitative effects of varying total copper composition rather than with effects of varying the form in which the copper is present at a constant copper composition. Furthermore, many of the prior glasses employed in such studies were very high melting materials which made it difficult, if not impossible, to accurately control oxidation and reduction reactions of copper in the glass.
It is known that copper, when present in solder glasses, exists in both the divalent and monovalent states and that with increasing temperature, the equilibrium Cu.sup.2.sup.+ .revreaction.Cu.sup.+ is shifted towards the right (see W. A. Weyl, Coloured Glasses, Chapter 11, Society of Glass Technology, Sheffield, 1951, pp. 154-167). In addition, once an equilibrium between the Cu.sup.+ /Cu.sup.2.sup.+ ratio is achieved at an elevated temperature in a molten glass, the ratio can be preserved at lower temperatures by quickly quenching the glass as discussed by Banerjee et al., "Thermodynamics of the System Cu-0 and Ruby Formation in Borate Glass", Journal American Ceramics Society, 57 (7) 286-90 (1974). Additional prior art discussions of copper in glasses can be found in P. L. White, "Mechanical Relaxations in Copper Alumino Silicate Glasses", Physical Chemistry Glasses, 12 (1) 11-14 (1971); P. L. White, "Mechanical Relaxation of Lithium-Copper Alumino Silicate Glasses", Physical Chemistry Glasses, 12 (4) 109-10 (1971); Ram et al., "New Conception on the Origin of Color in Copper Ruby Glass", Sprechsall, 102 (9) 315-20 (1969); and Ram et al., "Viscosity of Copper Ruby Glass in and Below the Striking Range of Temperature", Glass Technology, 9 (1) 1-4 (1968).
Further prior art to the present invention, although not published prior to the filing date of the present application, is presented by Powell et al, "Thermal of Cu.sup.+.sup.+ in PbO-B.sub.2 O.sub.3 Solder Glasses and Effect of Cu.sup.+ /Cu.sup.+.sup.+ Ratio on Thermal Properties of pbO-B.sub.2 O.sub.3 Glass" and Frieser et al., IBM Technical Bulletin, Volume 18, No. 6, Nov. 1975. Powell et al., in said paper suggest fining of glasses containing copper oxide at various temperatures and determined that the ratio of Cu.sup.+ /Cu.sup.+.sup.+ affected certain thermal properties including the softening temperature of the glasses evaluated. Frieser et al. in said IBM Technical Disclosure Bulletin suggest a process to eliminate cuprous oxide precipitation in a copper-containing lead glass so that the predominant species in the glass in Cu.sup.+.sup.+ and not Cu.sup.+ by bubbling oxygen through the molten glass. Neither Powell et al. nor Frieser et al. suggest regulating softening point by regulating the Cu.sup.+ content.
However, although the use of copper in solder glasses to vary softening point, for example, is well known, no one has heretofore suggested a process for controlling Cu.sup.+ to regulate and/or predetermine the softening point of the solder glass without altering the coefficient of thermal expansion. Furthermore, it was not known that the softening point of the solder glass could be accurately predetermined by properly controlling Cu.sup.+ regardless of the ratio of the Cu.sup.+ /Cu.sup.2.sup.+.
Accordingly, an object of the present invention is to provide a process for controlling the softening point of solder glass which contains copper oxide. Furthermore, an object of the present invention is to provide a process which makes it possible to preselect the softening point of solder glass containing copper oxide.
A further object of the present invention is to minimize the lot-by-lot scattering of the softening point of copper oxide containing glasses and to "tailor make" the softening point of a copper oxide containing glass of a single composition for a particular use.
The present invention makes it possible to obtain a preselected softening point of solder glass without changing the batch composition used to prepare the solder glass. Furthermore, the present invention makes it possible to obtain many different glass compositions having a range of different preselected softening points starting from only a single batch composition.
For instance, solder glass cane can be made for use in a gas discharge display panel sealing which can be designed to accommodate sealing conditions determined by other factors such as crazing problems without the necessity of changing the batch composition of the seal glass and its coefficient of thermal expansion.
The process of the present invention also makes it possible to reduce the lot-to-lot scattering which may result from prior unknown processing history of the glass such as different starting materials, unknown pulverizing conditions, unknown temperatures of melting, etc.